The present invention relates to an optical holographic apparatus for recording and reconstructing optical holograms in optical information processing or display technology. The present invention also relates to an optical interconnection apparatus for effective switching of optical paths in optical information processing, optical communication and optical measurement. The present invention further relates to an apparatus for applying optical correlation processing to a two-dimensional image obtained from an image sensor such as a CCD camera to effect automatic pattern recognition or measurement in the field of optical information processing and optical measurement. The present invention still further relates to a method of driving a holographic application apparatus for applying optical correlation processing to a two-dimensional image obtained from an imaging device such as a CCD camera to effect automatic pattern recognition and measurement in the holographic application field of optical information processing and optical measurement, or for reconstruction from a holographic image.
Conventionally, much effort has been made to realize a real-time hologram by using a light addressed liquid crystal light valve. The light addressed liquid crystal light valve mainly utilizes twist nematic liquid crystal (TN type liquid crystal). Further, the conventional light addressed liquid crystal light valve utilizes a photoconductive layer composed of bismuth silicate crystal (Bi.sub.12 SiO.sub.20 crystal) to record a hologram in order to reduce the wave number pitch of hologram interference fringes and to improve recording density and contrast of the reconstructed image, thereby achieving a recording density of 50-60 lp/mm and a reconstructed image contrast of 1:30, as disclosed in A. A. Vasil'ev et al., Sov. J. Quantum Electron. 14(2), Feb. 276-277 (1984).
However, the holographic apparatus utilizing the conventional light addressed liquid crystal light valve has a slow recording and reproducing speed of about several hundreds msec and has an insufficient contrast of the reconstructed image. Further, the conventional apparatus is difficult to handle because the light addressed liquid crystal light valve has to be stored in a dark space while applying thereto a voltage in order to maintain a hologram for a long time. Moreover, there is another problem in that an extremely large interferometer is needed due to a small angle between the reference light and signal light.
Conventionally, the optical interconnection has been studied as a basically important technology in the field of optical information processing, optical communication and optical measurement. Optical information processing is a key technology to such applications as optical interconnection between OEICs and interconnection between neurons in a neural network. Such interconnection has been realized by using holograms formed by silver salt photograph, thermoplastic or nonlinear optical crystal such as BaTiO.sub.3 single crystal. In the field of optical communication and optical measurement, generally the switching of the optical path and spectrometer are carried out with a mirror, a half mirror prism or a diffraction grating. Further, in the optical communication field, holography, as mentioned above in the optical information processing, can be utilized for optical interchanging.
However, with regard to the conventional optical separating element such as a mirror, a half mirror prism and a diffraction grating, generally the light path is fixed, and selective switching of the light path is carried out by mechanical means, thereby resulting in low switching speed and difficult adjustment. Holograms using the silver salt photographic plate have a similar problem in that the switching of the light path is difficult. With regard to holograms using thermoplastics or nonlinear optical crystal such as BaTiO.sub.3 single crystal, selective switching of the light path is possible, but thermoplastics need a great driving current and have a long response time in the order of at least several hundreds msec. The BaTiO.sub.3 single crystal is operated in the temperature range of about 20.degree. C.-130.degree. C., and therefore it cannot be used in a lower temperature range. Further, it has problems such as the size of the crystal is limited and the crystal is rather expensive.
Conventionally, the optical correlator using a Fourier transform hologram (as a matched filter) features a high S/N ratio, hence it has been frequently used for the study of pattern recognition and optical computing. Generally, a photographic dry plate is utilized to produce a Fourier transform hologram because of its high resolution and wide dynamic range. Namely, a Fourier hologram of the codes or reference image is recorded and developed on the dry plate. However, this method cannot rewrite code images in real-time. FIG. 13 shows an optical correlator utilizing an optically writable TN liquid crystal spatial modulator operable to effect pattern recognition in real time.
In FIG. 13, a laser source 201 emits a light which is expanded by a beam expander 202 and then divided into two beams by a beam splitter 203. One of the two beams passing through the beam splitter 203 is again divided by a beam splitter 204 into two beams. One of the two beams passing through the beam splitter 204 illuminates a code image on a code plate 205 to form a corresponding coherent code image. Thereafter, the coherent code image is Fourier transformed by the first Fourier transform lens 206, and thereafter irradiates a writing face of a light addressed TN liquid crystal light valve 234 to thereby form a Fourier code image. On the other hand, the other beam reflected by the beam splitter 204 is again reflected by the first mirror 208 to thereafter irradiate the writing face of the TN liquid crystal light valve 234 in the form of a reference beam to interfere with the Fourier code image to form interference fringes. The interference fringes are then recorded on the TN liquid crystal light valve 234 in the form of a Fourier code hologram with grey scale. Further, the other beam reflected by the beam splitter 203 is sequentially reflected by the second mirror 209 and the third mirror 210, and irradiates an input image on the input plate 211 to convert the input image into a corresponding coherent input image. The coherent input image is Fourier transformed by the second Fourier transform lens 212 and then irradiates through a polarizing beam splitter 213 onto a reading face of the TN liquid crystal light valve 234 to form the Fourier input image. Consequently, the Fourier code hologram is read from the light addressed TN liquid crystal light valve 234. The read image is then Fourier transformed by the third Fourier transform lens 214 to form the correlation image which contains a correlation function and a convolution function between the code image and the input image and a zero-order light. The intensity of the correlation function is detected by a photodetector 215 so as to effect recognition.
In such construction, the code plate 205 is placed on the front focal plane of the first Fourier transform lens 206. The light addressed TN liquid crystal light valve 234 is placed on the back focal plane of the first Fourier transform lens 206, and on the back focal plane of the second Fourier transform lens 212, and on the front focal plane of the third Fourier transform lens 214. The input plate 211 is placed on the front focal plane of the second Fourier transform lens 212. The photodetector 21 is placed on the back focal plane of the third Fourier transform lens 214.
In the FIG. 13 structure, the light addressed TN liquid crystal light valve 234 can be replaced by a bismuth silicate (Bi.sub.12 SiO.sub.20) crystal or a lithium niobate (LiNbO.sub.3) crystal which constitutes a light addressed spatial light modulator operative according to pockels effect of electro-optic crystal or photoconducting effect. Further, in place of the light addressed spatial light modulator, an imaging device such as a CCD camera is utilized to convert the Fourier code hologram into a corresponding electric signal, which is then displayed on a liquid crystal television or a magneto-optic spatial light modulator, as disclosed, for example, in H. K. Liu, J. A. Davis and R. A. Lilly, Optics Letters, Vol. 10, No. 12, 1988 and in D. L Flannery, J. S. Loomis and M. E. Milkovich, Applied Optics, Vol. 27, No. 19, 1988.
However, with regard to the conventional optical correlator utilizing a light addressed spatial light modulator as a matched filter of a Fourier code hologram, the light addressed spatial light modulator has relatively low resolution. Further, though the image can be recorded in gray scale, the dynamic range is rather narrow so that a complicated Fourier hologram cannot be recorded and the S/N ration of the pattern recognition is low. The speed of the pattern recognition is in the order of several hundred msec for rewriting the Fourier code holograms, which is not practically sufficient.
With regard to the other conventional optical correlator utilizing an electrically addressed spatial light modulator as a matched filter of a Fourier code hologram, the Fourier code hologram can be binarized to improve the S/N ratio and the rewriting speed of the Fourier code holograms is on the order of several ten msec to achieve fast operation; however, the resolution is relatively low, on the order of several lp/mm, such that complicated image recognition cannot be effected.
Conventionally, a hologram recording medium is comprised of a light addressed TN liquid crystal light valve in a holographic application apparatus such as an optical correlator utilizing a Fourier hologram as a matched filter, and a holographic device for reconstructing a holographic input image. However, the conventional light addressed TN liquid crystal light valve has relatively low resolution. Further, though the image can be recorded in gray scale, the dynamic range is rather narrow so that a complicated Fourier hologram cannot be recorded and the S/N ration of the pattern recognition is low. The speed of the pattern recognition is on the order of several hundred msec for rewriting of the Fourier code holograms, which is not practically sufficient.